The present invention relates to an attachment for a mechanical ventilator that permits a patient whose breathing is being assisted by the ventilator to be weaned therefrom. A mechanical ventilator is a device employed during the treatment of acute respiratory insufficiency to mechanically assist patient breathing by supplying a patient connected to the machine with a predetermined quantity of air at a predetermined pressure and at predetermined intervals. The patient may be connected to the machine via a trachael or tracheostomy tube or via a mask. The air supplied to the patient is usually ambient room air that has been enriched with oxygen and humidified by the ventilator. Typically, the selected volume of air is delivered to the patient at the beginning of each spontaneous inspiration or at fixed intervals if the patient's inspirational effort is insufficient to trigger the machine.
Ultimately, assuming normal patient recovery, the patient must be gradually disconnected from the ventilator and retaught to breathe spontaneously. For many patients, this "weaning" process is a very trying experience, one that they approach with much anxiety because of their fear of not being able to breathe without the assistance of the ventilator. For some patients, this anxiety and fear renders them uncooperative and unwilling to tolerate the often prolonged trials of spontaneous, unassisted breathing.
It was to solve this problem of patient weaning that the concept of intermittent mandatory ventilation (IMV) was developed. As disclosed in "Intermittent Mandatory Ventilation: A New Approach to Weaning Patients from Mechanical Ventilators" by John B. Downs et al., available from John B. Downs, M.D., NIH Research Fellow, Department of Anesthesiology, University of Florida College of Medicine, Gainesville, Florida, 32601, and "Methods of Administering Intermittent Mandatory Ventilation (IMV)," 19 Respiratory Care 187, March 1974, IMV is a method of automatic weaning whereby the ventilator is intermittently cycled to deliver a single volume of air to the patient at adjustably lengthening intervals. During the periods when air is not being supplied by the ventilator, the patient breathes spontaneously from a continuous alternate source of oxygen-enriched and humidified air. As the patient's spontaneous breathing improves, the periods between delivery of ventilator-supplied air are lengthened until, finally, the patient achieves continuous, unassisted, spontaneous breathing.
As disclosed in the second reference listed above, IMV systems are currently of two types: ambient reservoir and pressure reservoir. The ambient reservoir system employs a length of open-ended tubing to capture the air from the continuous source during periods of mandatory ventilation. The pressure reservoir system employs a small volume anesthesia bag in place of the length of open-ended tubing to capture and retain at a pressure slightly above ambient the air from the continuous source during the mandatory ventilation periods. In either system the captured air is available to the patient during spontaneous inspiration. However, with the ambient reservoir system there is a probability that the patient will inhale more air than the reservoir has captured and, therefore, an unknown quantity of non-oxygen-enriched and humidified air from the atmosphere. Such possibility is precluded by the pressure reservoir system.
To monitor the patient's progress during the weaning process, several physiological parameters must be continuously measured and analyzed. Among these parameters is the volume of air, known as the tidal volume, that is inspired and expired during each respiration. The usual method of measuring the tidal volume is to measure the volumetric flow through the patient's expiration tube. However, with the addition of the pressure reservoir to the continuous alternate air supply, the tidal volume cannot be determined accurately by measuring this volumetric flow because, during expiration, the expiration tube will contain air flowing from the alternate air source as well as the patient's exhalation. Consequently, the tidal volume measurement must presently be made by tapping the patient's connection tube close to the patient and before the tube branches into its expiration and inspiration components, such method being inconvenient and troublesome.
In addition, with current IMV valving and tubing arrangements which include one-way valves in both the expiration tube and the tube connecting the continuous alternate air supply to the patient, there is a substantial amount of "dead space" into which exhaled air from the patient can accumulate during each expiration. This exhaled air is then rebreathed during the next inspiration, decreasing the volume of oxygen-enriched air actually breathed by the patient and thus limiting the degree to which the quality of the air inspired by the patient can be controlled.